Aha. I believe this is the primary ticket for this issue: Unreal Engine Issues and Bug Tracker (UE-166377)
According to this post (which the commenter has also posted in a few other relevant threads I found) the issue is due to this commit. The first comment matches my experience (that it’s flushing the input). I guess I will try to build from source so I can attempt to revert that change.
I notice the commit description mentions a Jira ticket with FORT in the name, so I guess the change was made because of a Fortnite issue. I’ve only played Fortnite once, back before it was a BR, so I can’t say whether they make use of in-world UI elements and WidgetInteraction components.